Lens Material And Design Flashcards
1
Q
Does glass have a low or high abbe number?, what does this mean regarding chromatic aberrations
A
High abbe number=low chromatic aberrations
2
Q
What are 4 advantages of glass?
A
- highly transparent and colorless
- resistant to heat and chemicals
- high scratch resistant
- high abbe number= low aberrations
3
Q
What are 5 disadvantages of glass?
A
- weight
- poor impact resistance
- poor UV protection
- cant be tinted after manufacturing
- lengthy processing time
4
Q
What are the 2 types of plastics?
A
- thermoplastic (polycarbonate)
- thermosetting (CR-39)
5
Q
This plastic can be softened when heated?
A
Polycarbonate
6
Q
This plastic when hardened cant be softened even at high temps?
A
CR-39
7
Q
Does CR-39 have a love or high abbe number?
A
High
8
Q
What are the 5 advantages of CR-39?
A
- lighter weight than glass
- high abbe number
- inexpensive
- tintable
- many designs
9
Q
What are 4 disadvantages of CR-39?
A
- scratches and chips easily
- thicker than hi index plastic
- warpage
- not for children
10
Q
T/F: the higher the index, the thinner a lens can be and still produce the same power
A
True
11
Q
This is type of lens material is good for high prescriptions
A
Hi-index plastic
12
Q
Does hi index plastic have a low or high abbe number?
A
Low
13
Q
Since hi-index plastic has a low abbe number=more aberrations, what type of coating does it need?
A
A/R coating
14
Q
What is the refractive index of glass/crown glass?
A
1.523
15
Q
What is the refractive index of CR-39?
A
1.498
16
Q
What is the refractive index of polycarbonate?
A
1.586
17
Q
What are the 3 advantages of polycarbonate?
A
- excellent impact resistant
- UV protection
- many designs
18
Q
What are 4 disadvantages of polycarbonate?
A
- not great for optics
- not great for tinting
- soft surfaces, scratches easily if not treated
- must avoid exposure to heat
19
Q
This type of lens material is used in children’s glasses or sportswear
A
Polycarbonate
20
Q
What is the refractive index of trivex?
A
1.53
21
Q
What is the order of refractive indexes from low to high?
A
- ) CR39
- ) glass
- ) trivex
- )polycarbonate
- ) hi index plastic
22
Q
What are 5 advantages of trivex?
A
- tintable
- good UV protection
- very lightweight
- extremely high impact resistant
- fewer heat issues that polycarb
23
Q
This lens material is low cost and has excellent optics
A
CR-39
24
Q
This lens material is chemically resistant and not commonly used
A
Glass
25
This lens material is expensive, has chromatic dispersions, scratches easily, and is used for strong rx'
Hi-index plastic
26
These lens materials are used for children, sports, hazardous jobs, and thinner/light lenses
Polycarbonate and trivex
27
A point on the datum line halfway between the 2 vertical lines which are tangent to the edges
Geometrical center
28
Horizontal length of the box when referring to the frame or lens
Eye size or lens size (A)
29
The vertical length of the box
B distance (B)
30
Shortest horizontal distance between the lenses
Bridge size (DBL)
31
Horizontal distance between the geometrical centers of the 2 lenses
Frame PD
32
The longest diameter of a lens
Effective diameter (ED)
33
Point on the lens through which the line of sight passes (this would correspond to the optical axis if no prism power were needed)
Major reference point (MRP)
34
The plus power in a multifocal lens segment that is added to the power in the distance portion of the lens
Near addition
35
Distance is measured at ~___ft or ~____m
20ft or 6m
36
Computer/intermediate is measured at ____
Arms length
37
Reading is measured at ___cm
40cm
38
If asphericity is applied to the front surface of a plus lens, the surface will become ___ away from the center.
Flatter
39
If asphericity is applied to the back surface of a plus lens, the surface will become ___ away from the center.
Steeper
40
If asphericity is applied to the front surface of a minus lens, the surface will become ___ away from the center.
Steeper
41
If asphericity is applied to the back surface of a minus lens, the surface will become ___ away from the center.
Flatter
42
Designed for one rx correction
Single vision lenses
43
A lens where blurring in parts around the periphery results in a lack of clarity
Spherical lens
44
A lens with peripheral blurring improves, resulting in enhanced clarity
Aspheric lens
45
Longest horizontal dimension of the segment
Seg width
46
Longest vertical dimension of the segment
Seg depth
47
Distance from the lowest point on the lens to the top of the segment
Seg height
48
Vertical distance between the MRP and the top of the seg
Seg drop
49
T/F: the distance portion of the lens is decentered from the GC to correspond to the wearers PD
True
50
the distance from the Geometric Center (GC) to the Major Reference Point (MRP)
Distance decentration
51
the inset accounting for the near PD (because PD is different for near vision). The distance from the MRP to the center or the seg
Seg Inset
52
The inset of the seg as measured from the Optical Center of the lens. Distance from the GC to the center of the seg
total inset
53
First one-piece bifocal. Designed by starting with required near power and adding a second curvature to upper portion and back of surface.
Solid upcurve
54
Small segment cemented to back surface of lens. Segment could be replaced as add changed.
Cement
55
components fused together by heat instead of cement.
Fused kryptok
56
One-piece back surface bifocal
Ultex
57
What are the 7 types of bifocals
- straight top/flat top
- rounded
- blended
- executive
- curved top
- B or ribbon
- ultex
58
What are the 2 advantages of a round segment?
- least visible
| - light
59
What are the 2 disadvantages of a round segment?
- takes a while to get down to a useful width
| - most jump, especially in larger segs
60
What are the 3 advantages of a flat top?
- Immediate broad field
- minimal jump
- no prism
- light
61
What an disadvantage of a flat top bifocal?
Visible ledge in larger segs
62
What is an advantage of an executive bifocal?
Widest field
63
What are 6 disadvantages of an executive bifocal?
- thick
- heavy
- ugly
- collects debris
- chips
- no jump
64
has a smooth continuous building of power from the distance to the near through a “corridor”.
PAL
65
Do PAL have an image jump?
NO
66
process by which a PAL can be made thinner by removing some base up prism. This leaves residual BD prism and a substantially thinner lens
Prism thinning
67
the cylindrical power lateral to the umbilic of a progressive surface increases twice as rapidly as the addition power
Minkwitzs theorem
68
Vertical meridian of a PAL that is spherical at every point
Umbilic
69
T/F: lens designs with shorter corridors produce more unwanted cylinder power in the periphery or narrower viewing zones.
True
70
Design characterized by wider area of stable optics at distance and near along with a shorter, narrower corridor (steeper gradients); Better for sustained viewing tasks
Hard designs
71
Designs have narrower areas of optimal power at distance and near, but offer a longer, wider intermediate zone (shallower gradients); Better for dynamic viewing tasks
Soft designs
72
Many lenses today are multi-design meaning what?
Softer with decreasing add power and harder with increasing add power
73
PALS with small B measurements
Short corridor PALs
74
uses computer- aided design and processing to create high-level, customized lenses unique to your prescription; makes you see better and across more of your lens
Free-form processing
75
What are 4 advantages of PALs?
- no image jump
- cosmetically appealing
- more youthful and fashionable
- uninterrupted vision from distance to near
76
What are 3 disadvantages of PALs?
- expensive
- adaptation time
- unwanted Astigmatism is periphery
